The present invention relates to a cylinder head gasket placed between two members such as a cylinder head and a cylinder block of an internal combustion engine.
When a joint surface between a cylinder head and a cylinder block (cylinder body) of an automobile engine is sealed, a metal cylinder head gasket is placed between the cylinder head and the cylinder block to seal combustion gas, coolant water, and lubrication oil.
Such a cylinder head gasket has been changed from a lamenting type having many layers of metal plates to a simple structured cylinder gasket type composed of one or two metal plates for reducing a weight and production cost of an engine. Since only one or two structural plates are used, usable materials are limited for reducing a weight of an engine. A type and the number of sealing methods are limited, so that it is necessary to use a relatively simple sealing method.
As a weight and size of an engine have been reduced recently, the engine tends to have lower rigidity. Accordingly, it is difficult to obtain an even surface pressure around a cylinder bore for sealing a cylinder head. That is, because of a structural problem around a cylinder bore, when the gasket is tightened, it is difficult to generate an enough sealing surface pressure at a low rigid portion, thereby causing gas leak at the portion.
When an engine has a smaller size in a longitudinal direction to reduce a size thereof, a space between the cylinder bores decreases, thereby increasing a temperature of the cylinder bores. As a result, the cylinder head and the cylinder block are deformed, thereby increasing a chance in which gas leaks between the cylinder bores. When a small amount of gas leaks and enters a water hole or oil hole, gas enters liquid such as water and oil, thereby causing a problem in circulating water and oil and cooling the engine. Since this causes a problem for the engine, it is necessary to provide a counter measure.
A metal gasket wherein beads are formed around cylinder bores (combustion room holes) has been proposed. The beads are connected or cross between the cylinder bores, and have straight portions between the cylinder bores. An edge of the bead at a side of the cylinder bore is formed in a curved shape smoothly connecting the straight portion between the cylinder bores and arc portions around the cylinder bores except the straight portions (see Patent Reference No. 1).
With this structure, a decrease in the surface pressure at the bead crossing section is prevented for a small cylinder bore for a small engine. Also, a portion in which rigidity of the bead changes greatly around the bead crossing section is eliminated, thereby preventing gas leak at the bead crossing section.
However, in such a single bead structure, it may not be an enough solution for an increased temperature of the cylinder bore and lower rigidity of a recent engine.
Patent Reference; Japanese Patent Publication (Kokai) No. 2000-356266
In view of the problems described above, an object of the present invention is to prove a cylinder head gasket for effectively preventing gas leak between cylinder bores of an engine with a small size and low rigidity.
Further objects and advantages of the invention will be apparent from the following description of the invention.